The present invention relates to an image data compression system for use in facsimile or the like which encodes into a compressed code a binary or multilevel (gray scale) image signal obtained by main and sub-scanning of an image.
Heretofore there have been employed for the transmission and storage of a binary image signal one-dimensional runlength coding methods (e.g. Modified Huffman method and Wyle method) which use, as an information source, the number of successive pels (picture elements) of the same level in the main scanning of the binary image signal, i.e. what is called a runlength, an edge difference coding method (Modified Read method) which uses, as an information source, the difference between run changing points on a coding line (a main scanning line) and an immediately preceding line, and a predictive coding method. With these coding methods, the runlength in the one-dimensional runlength coding method, the difference between the run changing points in the edge difference coding method and the position of a reference pel in the predictive coding method are all dependent on the physical property, for example, the scanning density of the device used. However, some of binary image input/output device, such as facsimile apparatus and raster scan type CRT display, differ in the number of pels in the main scanning direction and the line density in the sub-scanning direction. If the same binary image encoded by the conventional coding method is provided to such devices of different line densities, then images displayed by them differ in size in horizontal and vertical directions, or partly drop out.
As a solution to this problem there has been proposed a system for matching, enlarging or reducing the size of the binary image through the use of a line density conversion system (for example, R. A. Ulichney et al., "Scaling Binary Images with the Telescoping Template", IEEE Trans. Patterns Analysis and Machine Intelligence, Vol. PAM1-4, No. 3, May 1982, p. 331-335). For the mutual conversion of binary image signals between devices of certain line densities, however, it is necessary to preknow their line densities. Accordingly, for the mutual conversion of binary image signals for transmission and reception by various types of input/output device, it is necessary to exchange information between the devices concerned on their line densities in advance and to prepare a plurality of conversion algorithms.
Also there has been proposed a method in which the number of the total pels in a picture is always set to a standardized value and encoded, as in facsimile, but this method is defective in that the matching property with geometric graphical forms and the extensibility for new functions are poor. Furthermore, according to the conventional methods, a program for inputting and outputting a binary patterns is implemented as a program dependent on the physical properties of the input/output devices, so that the portability of the program cannot be achieved.
Conventional coding methods for a half tone facsimile signal and a color image signal also have drawbacks similar to those referred to above in connection with the binary image signal.